The use of taper-threaded pipe and fittings is standard practice in the construction of small diameter (generally, ¼ inch through 2 inch) piping systems. The tapered screw thread gives rise, as the joint is tightened, to interference between the pipe and fitting that acts to prevent leakage and assure a rigid connection.
Lengths of pipe and associated fittings may be screwed together “by hand” until the imaginary conical pitch surfaces of the male and female threads meet. Further tightening, ranging from one half to one full pitch of the thread is then necessary to create sufficient interference between the threads to complete assembly of the joint. Although this method of construction provides a strong mechanical connection and one that can be made leak-tight, it has shortcomings that make its use troublesome in certain situations.
The geometry of taper pipe threads is governed by a pair of gauges, a ring gauge for male threads and a plug gauge for female threads. An acceptable thread is one that falls between one turn short of the gauge plane and one turn beyond the gauge plane, an axial span of two times the thread pitch. As a result, the overall length of a piping assembly can vary over a span of four times the pitch of the thread (2 times the pitch for the male thread, 2 times for the female) for every joint.
For example, the pitch of U.S. standard 1 inch pipe thread is 0.087 of an inch, the variability in length of an assembly of 1 inch pipe and fittings, therefore, is 0.087 of an inch×4 equals 0.35 of an inch per joint, this without considering the probability of further variation arising from tightening of the joints.
The high degree of uncertainty in predicting the completed length of piping assemblies poses a serious problem in situations wherein a piping a subassembly must meet mating connections at fixed positions within a piping system.
A further problem is encountered when a taper thread fitting, for instance a valve, is to be assembled into a piping system so that the stem will reside in a certain position when the assembly is completed. To accomplish this, the fully tightened condition must be achieved just as the stem arrives in the desired position. Unfortunately, with conventional taper threaded pipe and fittings, the coincidence of full pipe “makeup” and proper positioning is entirely happenstance.
Fittings are customarily over-tightened in an effort to achieve a desired orientation, resulting in possible damage including stripping of the threads. Under-tightening invites leakage and inadvertent repositioning (usually loosening) of the fitting through normal use.
Assembly of taper-thread piping requires the application of considerable torque to achieve sound joints. Application of the necessary torque can be detrimental to other components of the system that must provide a reaction to oppose the applied torque. Furthermore, applying this torque requires use of large tools that are difficult to maneuver in confined areas.
Also cutting pipe threads in a section of pipe, or in a fitting requires a substantial amount of material, i.e., the wall thickness of the pipe must be sufficient to allow for thread cutting without significant weakening of the pipe. In certain cases where thin walled pipe could be used, for example in relative lower pressure systems, a heavier gauge, and more expensive, pipe must be used solely for the purpose of supporting the thread cutting.